text: Coccolithophore biomass is controlled by a combination of bottom-up (physical–biogeochemical environment) and top-down factors (predator–prey interactions), but the relative importance of the two has not yet been assessed for coccolithophores in the Southern Ocean. Bottom-up factors directly impact phytoplankton growth, and diatoms and coccolithophores are traditionally discriminated based on their differing requirements for nutrients, turbulence, and light. Based on this, Margalef's mandala predicts a seasonal succession from diatoms to coccolithophores as light levels increase and nutrient levels decline. In situ studies assessing Southern Ocean coccolithophore biogeography have found coccolithophores under various environmental conditions, thus suggesting a wide ecological niche, but all of the mentioned studies have almost exclusively focused on bottom-up controls.
  However, phytoplankton growth rates do not necessarily covary with biomass accumulation rates. Using satellite data from the North Atlantic, Behrenfeld stressed in 2014 the importance of simultaneously considering bottom-up and top-down factors when assessing seasonal phytoplankton biomass dynamics and the succession of different phytoplankton types owing to the spatially and temporally varying relative importance of the physical–biogeochemical and the biological environment.
  In the Southern Ocean, previous studies have shown zooplankton grazing to control total phytoplankton biomass, phytoplankton community composition, and ecosystem structure, suggesting that top-down control might also be an important driver for the relative abundance of coccolithophores and diatoms. But the role of zooplankton grazing in current Earth system models is not well considered, and the impact of different grazing formulations on phytoplankton biogeography and diversity is subject to ongoing research.
  The diagram on the left shows the spatial distribution of different types of marine sediments in the Southern Ocean. The greenish area south of the Polar Front shows the extension of the subpolar opal belt where sediments have a significant portion of silicous plankton frustules. Sediments near Antarctica mainly consist of glacial debris in any grain size eroded and delivered by the Antarctic Ice.